Residential gas furnace products have means for the detection of combustion during all operating cycles of the system. Fail-safe operation of these detection systems is of paramount importance to safety and system reliability. There can be no condition in which the flame sensing unit, i.e. the photodiode or flame rod, produces a false flame response to the input of the flame sense circuitry. The system controller should know if the flame sensor circuitry has failed in a constant flame-on condition. A no-flame signal to the system controller, when there is a flame, is not a safety problem and therefore is permissible. In addition, the flame sensing system should be reliable over time.
Prior art flame detection systems use either a photosensor or an ion probe to detect the presence of a flame, together with logic circuitry to process and analyze the detector output. Ion probe detectors are placed in contact with the flame, thus being subject to deposition and corrosion that may interfere with their operation. An optical flame sensor, such as a photodiode, is non-intrusive, thus enabling it to view the flame without being subject to these detrimental processes. Deposition by insulating materials produced from high temperature sealants used in gas furnaces is common.
Prior art photodiodes operated in the photoconductive mode operate with reverse bias. In this mode, excessive diode leakage (referred to as "dark current") resulting from, for example, but not limited to, a poor device, or elevated temperature, can cause the circuitry to give a false indication of a flame-on condition. Prior art photodiodes operating in the photovoltaic mode use no external bias across the photodiode, resulting in no dark current, increased sensitivity to low light levels, and slightly lower responsivity at longer wavelengths. However, the photo-generated voltage is a logarithmic function of incident light intensity for open circuit photovoltaic operation. Specifically, due to the logarithmic response, the signal produced by a hot surface ignitor, which is used to ignite the main gas flame, is difficult to discern from the signal produced by the flame.
For example, U.S. Pat. No. 4,322,723 appears to disclose a photosensor to detect the presence of a gas flame, but the logarithmic and transconductance amplifiers disclosed have difficulty discerning between the ignitor signal and flame signal. U.S. Pat. No. 4,039,844 appears to disclose a silicon photodiode connected to an a.c. coupled transconductance amplifier; however, the overall circuit is extremely complex, requires operator gain adjustment and does not appear failsafe. Furthermore, the photodiode requires an undesirably high signal level on the order of 1-500 microamperes, indicating a high level of light intensity.